The libstdc++so7 package contains a preview of the GNU Standard C++ Library from
libstdcxx_so_7-branch branch. It is considered experimental, and may in fact be
completely unreliable. It is unsupported, needless to say. Don't rely on its
ABI, or its API as it might change at any time. To use, invoke
'g++-libstdc++so_7' instead of 'g++'.